1. Field of Invention
This invention relates to chemical mechanical polishing apparatus, and more particularly to the technique of optical polishing endpoint detection. The invention provides an apparatus for optical endpoint detection that avoids contamination of the detector""s optical fiber tip and resultant endpoint errors.
2. Description of the Related Art
Chemical mechanical polishing (CMP) has emerged as a crucial semiconductor technology, particularly for devices with critical dimensions smaller than 0.5 micron. One important aspect of CMP is endpoint detection (EPD), i.e., determining during the polishing process when to terminate the polishing.
Many users prefer EDP systems that are xe2x80x9cin situ EPD systemsxe2x80x9d, which provide EPD during the polishing process. Numerous in situ EPD methods have been proposed, but few have been successfully demonstrated in a manufacturing environment and even fewer have proved sufficiently robust for routine production use.
One group of prior art in situ EPD techniques involves the electrical measurement of changes in the capacitance, the impedance, or the conductivity of the wafer and calculating the endpoint based on an analysis of this data. To date, these particular electrically based approaches to EPD are not commercially available.
One other electrical approach that has proved production worthy is to sense changes in the friction between the wafer being polished and the polish pad. Such measurements are done by sensing changes in the motor current. These systems use a global approach, i.e., the measured signal assesses the entire wafer surface. Thus, these systems do not obtain specific data about localized regions. Further, this method works best for EPD for tungsten CMP because of the dissimilar coefficient of friction between the polish pad and the tungsten-titanium nitride-titanium film stack versus the polish pad and the dielectric underneath the metal. However, with advanced interconnection conductors, such as copper (Cu), the associated barrier metals, e.g., tantalum or tantalum nitride, may have a coefficient of friction that is similar to the underlying dielectric. The motor current approach relies on detecting the copper-tantalum nitride transition, then adding an overpolish time. Intrinsic process variations in the thickness and composition of the remaining film stack-layer mean that the final endpoint trigger time may be less precise than is desirable.
Another group of methods uses an acoustic approach. In a first acoustic approach, an acoustic transducer generates an acoustic signal that propagates through the surface layer(s) of the wafer being polished. Some reflection occurs at the interface between the layers, and a sensor positioned to detect the reflected signals can be used to determine the thickness of the topmost layer as it is polished. In a second acoustic approach, an acoustical sensor is used to detect the acoustical signals generated during CMP. Such signals have spectral and amplitude content that evolves during the course of the polish cycle. However, to date there has been no commercially available in situ endpoint detection system using acoustic methods to determine endpoint.
Finally, the present invention falls within the group of optical EPD systems. One approach for optical EPD systems is of the type disclosed in U.S. Pat. No. 5,433,651 to Lustig et al. in which a window is used to detect endpoint. However, the window complicates the CMP process because it presents to the wafer an inhomogeneity in the polish pad. Such a region can also accumulate slurry and polish debris.
Another approach is of the type disclosed in European application EP 0 824 995 A1, which uses a transparent window in the actual polish pad itself. A similar approach for rotational polishers is of the type disclosed in European application EP 0 738 561 A1, in which a pad with an optical window is used to transmit light used for EPD. In both of these approaches, various means for implementing a transparent window in a pad are discussed, but making measurements without a window were not considered. The methods and apparatuses disclosed in these patents require sensors to indicate the presence of a wafer in the field of view. Furthermore, integration times for data acquisition are constrained to the amount of time the window in the pad is under the wafer.
In another type of approach, the carrier is positioned on the edge of the platen so as to expose a portion of the wafer. A fiber optic based apparatus is used to direct light at the surface of the wafer, and spectral reflectance methods are used to analyze the signal. The drawback of this approach is that the process must be interrupted to position the wafer in such a way as to allow the optical signal to be gathered. In so doing, with the wafer positioned over the edge of the platen, the wafer is subjected to edge effects associated with the edge of the polish pad going across the wafer while the remaining portion of the wafer is completely exposed. An example of this type of approach is described in PCT application WO 98/05066.
In another approach, the wafer is lifted off of the pad a small amount, and a light beam is directed between the wafer and the slurry-coated pad. The light beam is incident at a small angle so that multiple reflections occur. The irregular topography on the wafer causes scattering, but if sufficient polishing is done prior to raising the carrier, then the wafer surface will be essentially flat and there will be very little scattering due to the topography on the wafer. An example of this type of approach is disclosed in U.S. Pat. No. 5,413,941. The difficulty with this type of approach is that the normal process cycle must be interrupted to make the measurement.
Yet another approach entails monitoring absorption of particular wavelengths in the infrared spectrum of a beam incident upon the backside of a wafer being polished so that the beam passes through the wafer from the nonpolished side of the wafer. Changes in the absorption within narrow, well-defined spectral windows correspond to changing thickness of specific types of films. This approach has the disadvantage that, as multiple metal layers are added to the wafer, the sensitivity of the signal decreases rapidly. One example of this type of approach is disclosed in U.S. Pat. No. 5,643,046.
One of the techniques for detecting when a silicon wafer surface has been polished to the extent required, is the use of optical endpoint detectors. Such detectors present several problems in practical application. In particular, optical endpoint detectors are subject to contamination of the sensor (optical fiber tip), resulting in errors or oversaturation as a result of receiving too much input light. The problems inherent in prior optical art endpoint detection systems may be better explained with reference to the attached FIGS. 1, 1A, and 2. FIG. 1 is a schematic representation, not to scale, of a common chemical mechanical polishing head, including a platen 10xe2x80x2 to which is mounted a central spindle 12xe2x80x2, for rotating the platen. The underface of the substantially circular platen 10xe2x80x2 is covered with a polishing pad 14xe2x80x2, that is attached by conventional means such as adhesives. As shown in FIG. 2, the polishing pad 14xe2x80x2 is often scored with grooves running in xe2x80x9cxxe2x80x9d and xe2x80x9cyxe2x80x9d directions to form a grid with parallel x-direction grooves 16xe2x80x2 and crossing perpendicular grooves 18xe2x80x2. These grooves are typically shallow and narrow and assist in the distribution of the chemical slurry during chemical mechanical polishing.
When an optical endpoint detector is used in the chemical mechanical polishing apparatus, an optical fiber 20xe2x80x2 is inserted through a bore in the platen 10xe2x80x2 and through a registering bore in pad 14xe2x80x2 so that the distal tip of the fiber is flush with the lower end of a groove 16xe2x80x2 and thus slightly spaced from the underside of pad 14xe2x80x2 by the groove depth, as schematically shown in FIG. 1A. Ordinarily, two optical fibers 20xe2x80x2 are usedxe2x80x94one to act as a xe2x80x9csend fiber,xe2x80x9d and the other a xe2x80x9creceive fiber.xe2x80x9d
It has been found that the above-described prior art optical endpoint detection system is subject to interference from contaminants resulting from workpiece polishing, and air bubbles that form in the chemical slurry and that intensify light received through the xe2x80x9creceive fiberxe2x80x9d 20xe2x80x2, sometimes resulting in oversaturation of its optical detector (not shown). As might be expected, during chemical mechanical polishing, especially of semiconductor wafers that include copper-based circuitry, copper particulates and reaction products of these particulates come into contact with optical fibers 20xe2x80x2, resulting in fouling and contamination of these fibers. As a result, precise optical endpoint detection is adversely affected.
There exists a need for an optical endpoint detection system that is simple, self-cleaning or easy to clean, and that may be retrofitted to existing chemical mechanical cleaning apparatus.
This summary of invention section is intended to introduce the reader to aspects of the invention and is not a complete description of the invention. Particular aspects of the invention are pointed out in other sections hereinbelow, and the invention is set forth in the appended claims which alone demarcate its scope.
The invention provides an apparatus for optical endpoint detection of a polishing process of a semiconductor wafer or other workpiece. The apparatus includes a hydrophobic light pipe in optical communication with the workpiece, and extending at least partially through the polishing pad of the apparatus. In certain embodiments, an optical fiber is in optical communication with an opposite end of the light pipe and is spaced from the polishing surface of the polishing pad, thereby preventing contamination of the optical fiber. In other embodiments, the light detector is in direct optical communication with the light pipe.
In a preferred embodiment, the light pipe is hydrophobic, i.e., the chemical slurry used in the chemical mechanical polishing process does not adhere to light pipe surfaces. Thus, the light pipe may be of any optically transparent substance, that is either hydrophobic or has a hydrophobic coating. The preferred hydrophobic light pipes are less subject to fouling with chemical mechanical polishing particulate debris and less susceptible to interaction with bubbles in chemical slurries that may cause light amplification with potential oversaturation of the light detector.
In another embodiment, a xe2x80x9cwindowxe2x80x9d is provided in a polishing pad, the window is substantially transparent to transmit sufficient light for optical endpoint detection, and is further also hydrophobic. As with the light pipes, the hydrophobic windows are less susceptible to fouling with polishing debris, preferably have a surface flush with the polishing pad""s polishing surface, and are less susceptible to interaction with bubbles that make cause light amplification and oversaturation of the light detector.